Systems for vibration isolation are known. For example, European Patent Specification EP 927830 B1 discloses a lithographic appliance which is mounted on air bearings for vibration isolation. Vibration isolation systems such as these are used in widely differing fields of engineering, in particular in the field of semiconductor manufacture and for high-resolution imaging systems such as MRI installations and electron beam microscopes.
As manufacturing precision increases, the requirements for vibration isolation systems such as these also become more stringent. Vibration isolation systems have therefore been developed which have active control via actuators in addition to mechanical decoupling of the mass to be isolated, for example by means of air bearings. In vibration isolation systems such as these, position changes of the mass to be isolated, in particular vibration, are detected via sensors. The position change is counteracted via actuators.
Electronic control systems make vibration isolation systems such as these even able to predict position changes caused by disturbance influences and to counteract a possible position change before it occurs.
Known vibration isolation systems such as these have been found to have the disadvantage that the mass to be isolated is regarded as a rigid body and it is not possible to take account of vibration originating from the body, such as natural-frequency vibration of isolated masses, in the control of the vibration isolation system. It has not been possible to include structural vibration such as this with previous control models. In fact, it has been found that any attempt to take account of structural vibration in the drive for the actuators of an active vibration isolation system leads to it not being possible to reduce the structural vibration, and in many cases it is even increased.
In known active vibration isolation systems, the mass to be isolated is therefore normally regarded as a rigid body and is masked out for control purposes, that is to say the vibration of the mass to be isolated itself is ignored in the calculation of compensation signals.